Osteoblasts have been implicated in the regulation of hematopoiesis and more recently, in the development of myeloid malignancies. A single activating mutation in ?-catenin signaling in osteoblasts is sufficient to induce acute myeloid leukemia (AML) development. The disease is transplantable and characterized by clonal evolution at the cytogenetic level suggesting the induction of driver mutations in HSCs. In humans, nuclear accumulation and increased ?- catenin signaling in osteoblasts was identified in 38% of patients with MDS/AML. These patients show complex cytogenetics, unfavorable prognosis, and resistance to standard treatment. However, a comprehensive characterization of driving genomic alterations and their specific molecular mechanisms leading to osteoblast-induced MDS and transformation to AML (OIAML) is missing. We have found that OIAML is associated with recurrent chromosomal aberrations in mll5, the mouse orthologue of the human KMT2E, translocations in which are associated with poor prognosis in AML patients. Moreover, AML patients with activated ?-catenin in their osteoblasts strongly associate to somatic mutations in TP53 in the tumor clone. Interestingly, in the TCGA AML cohort somatic losses of KMT2E are strongly associated with TP53 loss of function alterations suggesting that KMT2E and TP53 may act synergistically in osteoblast induced leukemias. However, it is not clear how TP53 or mll6 inactivation contributes to this process and whether there are other cooperating genomic alterations leading to leukemia development. We hypothesize that osteoblast-induced leukemias constitute a well-defined clinical and molecular entity, characterized in part by activating ?-catenin signaling in osteoblasts which subsequently instigates genetic events in HSCs that lead to their oncogenic transformation. Those include TP53 and KMT2E mutations in myeloid clones and other encompassing alterations. In this proposal we will dissect the synergy of ?-catenin-related genetic alterations in osteoblast-induced leukemias and characterize their role in development and evolution of human MDS/AML. We will also examine the role of TP53 and KMT2E inactivation and their potential synergism in ?- catenin related osteoblast -induced leukemias.